Aluminum nitride has high heat conductivity and an excellent electrical insulating property, and has been used as a high heat conductive substrate, a heat radiating component, an insulating/heat radiating filler, and the like. Recently, electronic semiconductor components such as integrated circuits (ICs) and computer processing units (CPUs) mounted on high-performance electronic devices, represented by a notebook computer, an information terminal, and the like, are being downsized and highly integrated, such that downsizing of heat radiating members has become essential. Examples of the heat radiating members include a heat radiating sheet and a film-like spacer in which a high heat conductive filler is filled in a matrix such as resin and rubber, a heat radiating grease in which a high heat conductive filler is mixed in silicone oil to have fluidity, and a heat radiating adhesive in which a high heat conductive filler is mixed in an epoxy resin.
Here, as the high heat conductive filler, aluminum nitride, boron nitride, alumina, magnesium oxide, silica, graphite, various metal powders, and the like are used.
In order to improve the heat conductivity of the heat radiating materials, however, it is important that a filler having a high heat conducting property is densely filled. For this reason, an aluminum nitride powder is required to have spherical aluminum nitride particles of a size of several microns to several tens of microns.
Generally, as methods for producing the aluminum nitride powder, a reductive nitridation method of firing alumina and carbon in a nitrogen atmosphere, a direct nitridation method of reacting metal aluminum directly with nitrogen, and a gas phase method of reacting alkylaluminum with ammonia followed by heating have been known.
However, it is difficult to form aluminum nitride powders by the reductive nitridation method and by the gas phase method to have spherical shapes, and their particle sizes are about a submicron size.
Meanwhile, the direct nitridation method makes it possible to relatively easily control the particle size and thus to obtain the aluminum nitride particles of several microns to several tens of microns. However, a pulverization step is essential in this method. Therefore, the particles of the obtained aluminum nitride powder have an angular or amorphous shape, which causes a decrease in fluidity, and it is difficult to very densely fill the aluminum nitride powder as a filler in the resin.
In view of the above, various studies have been made in order to obtain an aluminum nitride powder having a spherical shape and a desired average particle size.
For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. H3-23206) discloses a method for obtaining an aluminum nitride powder of an average particle size of not less than 3 μm and having a rounded shape by firing a mixture of an alumina powder and a carbon powder in an inert atmosphere to form an aluminum oxide, allowing particles thereof to grow and then firing (nitriding) the particles thereof in a non-oxidizing atmosphere containing nitrogen. However, there is a problem that since the aluminum nitride powder obtained by this method has an elliptical shape, has low sphericity, and involves conversion of the firing atmosphere, it is difficult to control the growth of alumina particles, that is, to control the particle size distribution of the obtained aluminum nitride powder.
In addition, Patent Document 2 (Japanese Unexamined Patent Application Publication No. H2005-162555) discloses a method for producing a spherical aluminum nitride powder having an average particle size of 50 μm or less, sphericity of 0.8 or more, and excellent water resistance, by reductively nitriding a spherical alumina by nitrogen gas or ammonia gas in the presence of carbon and then subjecting it to surface oxidation. However, in the above production method, since the spherical shape of alumina as a raw material is made into the shape of the aluminum nitride powder of the final product as it is, it is necessary to use alumina having a particle size equivalent to or larger than a desired particle size. In the reductive nitridation of alumina having such a large particle size, a long reaction time is required to improve its conversion.
In addition, Patent Document 3 (Japanese Unexamined Patent Application Publication No. H5-221618) discloses a method for producing an aluminum nitride powder by firing a mixed powder of an aluminum oxide powder, a carbon powder, and a rare earth compound as a starting material in a non-oxidizing atmosphere containing nitrogen. This method utilizes the function of accelerating the reaction of an alkaline earth metal compound or a rare earth compound to produce aluminum nitride at a low temperature of 1500° C. or lower. However, the aluminum nitride powder obtained by this method specifically has a particle size of only about 1 μm, and a relatively large particle size of about several microns is not obtained.
Further, Patent Document 4 (Japanese Unexamined Patent Application Publication No. H2002-179413) discloses a method of firing an aluminum nitride powder of amorphous particles in a flux including compounds of alkaline earth metals, rare earth metals, and the like so as to assume a spherical shape, and then dissolving the flux to isolate the aluminum nitride powder. This method makes it possible to obtain the aluminum nitride powder having excellent fluidity and filling properties. However, there is a problem that impurities such as oxygen and the like easily infiltrate into the aluminum nitride powder during the heat treatment process.
Furthermore, Patent Document 5 (Japanese Unexamined Patent Application Publication No. H11-269302) discloses a method for producing a spherical aluminum nitride powder by adding a forming assistant to an AlN (aluminum nitride) powder produced by a predetermined method, wet-pulverizing the mixture thereof, granulating the pulverized mixture by using a spray drier, mixing a BN powder into the obtained granulated product (granules), and then firing and sintering the mixture at a high temperature in a nitrogen atmosphere. According to this method, however, the firing is necessary for sintering the obtained particles in addition to firing for nitridation of aluminum, and the sintering temperature is very high. Thus, the firing must be carried out twice at high temperatures and its granulation is not easy. Further, since AlN is vulnerable to moisture, it cannot be subjected to spray drying by using water as a solvent, and thus another surface modification is required for the water-based spray drying.
To solve these problems, there has been suggested a method of conducting spray-drying using precursors such as Al2O3, Al(OH)3, and boehmite which are stable in a water system, mixing the dried product with carbon, followed by heat treatment at 1200° C. to 1800° C. in a nitrogen atmosphere. However, when an α-Al2O3 phase is used as a precursor, the AlN conversion is 100%, but when Al(OH)3 or boehmite and other Al2O3 phases are used as precursors, the AlN conversion is low, and thus there is a problem that the use of the precursor is limited.
Therefore, there is a great need for a production method capable of efficiently producing a spherical aluminum nitride powder having a spherical shape most suited for use as a filler, and having an average particle size of several tens of microns.